High sensitivity optical communication links are vital for the design of future high-performance communication networks. FIG. 1 provides an example of such a network. This network includes satellite nodes 105 having free-space optical transmission channels 110. The network 100 also includes ground-based network nodes 115, such as central offices. Between the ground-based network nodes 115 are fiber optic or free space transmission channels 120. Both the satellite nodes 105 and ground-based nodes 115 include transmitters and receivers (not shown).
It is well known that for good sensitivity, optical filters in the receivers need to be matched to the transmitted waveform. See H. L. Van Trees, Detection, Estimation and Modulation Theory, Part 1, pp. 1–15, 224–271, Wiley, N.Y. 1968 and P. S. Henry, “Error-Rate Performance of Optical Amplifiers,” in Proc. OFC '89, Houston, Tex., February 1989. Sensitive receiver performance reduces transmitter or mid-span amplifier requirements, extends link distances, and provides additional margin. See J. C. Livas, “High Sensitivity Optically Preamplified 10 Gb/s Receivers,” Proceedings of the Optical Fiber Communication Conference 1996, post deadline paper PD 4, 1996 and D. O. Caplan, M. L. Stevens, D. M Boroson, J. E. Kaufmann, “A Multi-Rate Optical Communications Architecture with High Sensitivity,” LEOS, November 1999. This is especially beneficial for free space communications since improvements in receiver sensitivity directly reduce transmitted power requirements.
Given the trend towards ultra-high speed 100 Gbps class all-optical networks, the need for all-optical filtering becomes more apparent as network elements increasingly surpass the capabilities of electronics. Therefore, processing in the optical domain becomes essential.
High sensitivity optical receivers are ultimately limited by shot noise that arises due to the variance in photon arrival times. The best performance that can be obtained in an optical communication link occurs when the shot or quantum noise is the dominant noise source. This is known as the quantum limit. For binary intensity modulation (IM) formats, such as on-off-keying (OOK) and binary pulse position modulation (PPM), using optically preamplified receivers, the quantum limited performance at 10−9 bits error rate (BER) corresponds to approximately 40 photons/bit or approximately −43 dBm (−50 nW) at 10 Gbps.
FIG. 2A is a schematic diagram of an optically preamplified on-off-keying (OOK) receiver 205. The receiver 205 includes an optical amplifier 206a, optical bandpass filter 206b, PIN-Diode photodetector 206c, electrical lowpass filter 206d, and decision circuitry 206e (collectively, stages 206). These stages 206 are typical of optical receivers.
Between each of the stages in the receiver 205 is a set of corresponding spectral diagrams 210 graphically representing optical or electrical spectral densities of signals processed by the corresponding components in the receiver 205. A spectrum 211a of a received optical signal is received by the optical amplifier 206a. A spectrum 211b of the amplified optical signal is outputted by the optical amplifier 206a and filtered by the optical bandpass filter 206b. A spectrum 211c of the filtered, amplified, optical signal is converted to an electrical frequency spectrum 211d by the PIN-photodiode photodetector 206c. The electrical frequency spectrum 211d is filtered by the lowpass filter 206d, producing a spectrum 211e that is processed by the decision circuitry 206e. 
FIG. 2B is a digital waveform 215 that graphically represents the resulting digital signal received by the decision circuitry, where the resulting digital signal includes noise (e.g., amplified spontaneous emission, ASE) superimposed on true and false logic levels of the digital waveform 215.
The following equations approximately describe the noise riding on the digital waveform 215.                Received signal current: Is=GehPs/hn        Received ASE current: Isp=ehPASE/hn=ehnsp(G−1)Bo        Received noise variance:                    −Nshot=2e(Is+Isp)Be            −Nsignal×ASE=4 G Is Isp Be/Bo            −NASE×ASE=Isp2Be(2Bo−Be)/Bo2             −NTot=Nshot+Nsignal×ASE+NASE×ASE                         SNR=(GehPs/hn)/(SQRT(NTot(“1”))+(SQRT(NTOT(“0”))))        BER˜Q[SNR]˜e(exp(−Q2/2))/(SQRT(2p)Q)˜½ e−SNR/2        Bo=2Be, G>>1, nsp=1, Receiver Sensitivity @ BER=10−9=>N=40 photons/bit        
Thus, at a bit error rate (BER) of 10−9, the theoretical sensitivity of the receiver 205 is 40 photons/bit. Detailed analysis with comparable results can be found in E. Desurvire, in Erbium Doped Fiber Amplifiers, pp. 155–187, John Wiley & Sons, New York, 1994 and S. B. Alexander, in Optical Communication Receiver Design, pp. 273–283, 292–310, SPIE Optical Engineering Press, Bellingham, Wash., USA, 1997.
High sensitivity quantum-limited optical receivers are particularly useful in free-space communications since they directly reduce the transmitter power required to close the link. Some examples include ship-to-shore communications and inter-building links that are sensitive to weather conditions, satellite cross-links, and deep space communications where distances/link budget can change significantly.
At high bit rates, optical preamplified receivers are the most sensitive receivers to date and in widespread use throughout the telecommunications industry.
As indicated by Shannon's Theorem, the capacity of a channel is a function of the bandwidth available and the signal-to-noise ratio (SNR). Shannon's Theorem (1949) says that, “error-free communications are possible up to rate C bits per second (bps) over a channel of bandwidth B (Hz) at a given signal-to-noise ratio (SNR),” and is expressed as: C=B log2(1+SNR). Shannon's Theorem motivated development of digital communications systems, including work at MIT Lincoln Laboratory from the early 1950's to the present.
Extensive efforts have been made throughout the telecommunications industry to expand the available capacity of optical networks—primarily by increasing the channel bandwidth (e.g., “S”, “C”, “L” bands, Raman amplifiers, etc.), as discussed in S. Kawai, H. Masuda, K. Suzuki, K. Aida, “Ultrawide, 75 nm 3 dB gain-band optical amplifier utilizing erbium-doped fluoride fiber and Raman fiber”, OFC '98., p. 32–33, February 1998; A. K. Srivastava, “Wide bandwidth high capacity systems”, OFC/IOOC '99, v. 4, p. 59–60, February 1999; S. Kinoshita, “Advances in optical fiber amplifiers for WDM systems”, APCC/OECC '99, v. 2, p. 1333–1334, October 1999; and A. E. Willner, “SNR analysis of crosstalk and filtering effects in an amplified multichannel direct-detection dense-WDM system”, IEEE Photonics Technology Letters, p: 186–189, v. 4, February 1992.
Alternatively, the channel capacity can also be increased by improving the SNR. Matched optical receivers maximize signal-to-noise ratio (SNR).